1. Field of the Invention
The present invention relates generally to a selective deposition of hydrous ruthenium oxide (RuO2 or RuO2.xH2O and/or RuOxHy) thin films. More particularly, the invention relates to the selective deposition of hydrous ruthenium oxide thin films by chemical vapor deposition (CVD) on selected or modified substrates. The hydrous ruthenium oxide films made by the method of the invention will be useful for catalytic and electronic applications.
2. Description of the Related Art
RuO2 is found in both anhydrous and hydrous forms. The anhydrous form of RuO2 exhibits the electrical properties of a metal. It is used as a thick film resistor and as an electrode material in capacitors because of its metallic conductivity and corrosion resistance. Anhydrous RuO2 thin films are also used as barrier coatings for some electronic applications, and in catalysis.
The hydrous form of RuO2 is written as RuO2.xH2O, RuO2, and/or RuOxHy. Hydrous ruthenium oxide can contain from 0.03 to 3 moles of structural H2O per mole of RuO2. The structural water in RuO2 can be changed by heating the material between 25 and 400xc2x0 C., see FIG. 4. Like the anhydrous form of RuO2, hydrous ruthenium oxide is a metallic conductor, and in addition, it also conducts protons. Hydrous ruthenium oxide has pseudocapacitive behavior. It is used as a charge-storage medium in ultracapacitors. Charge storage of hydrous ruthenium oxide is optimized when it is heated between 130 and 170xc2x0 C. and has the approximate formula RuO2.0.5H2O. The hydrous form of ruthenium oxide also has numerous applications in catalysis and electrocatalysis. When combined with platinum, hydrous ruthenium oxide is an excellent electrocatalyst for methanol oxidation and is therefore useful for direct methanol fuel cells. Thin films of hydrous ruthenium oxide are often deposited on Ti and/or TiO2 to make the dimensionally stable electrodes used for brine electrolysis by the chlor-alkali industry. Hydrous ruthenium oxide is used for water oxidation when combined with a strong oxidant, e.g., Ce4+, Ru(bpy)33+.
The deposition of hydrous and anhydrous ruthenium oxide thin films has been the subject of numerous publications and patents. Anhydrous thin films are typically deposited via CVD processes or physical methods, such as pulsed laser deposition. The hydrated forms of the films are deposited from aqueous solutions, usually using RuCl3 precursors. Most of the available processes must be carried out at elevated temperatures, i.e., greater than 300xc2x0 C., in order to complete the decomposition of the Ru precursors.
Previous research has shown that RuO4 is a precursor for the deposition of solid ruthenium oxide. For instance, the RuO4 that forms during nuclear fission processes volatilizes to RuO4(g) and deposits as a ruthenium oxide on the stainless-steel surfaces of the nuclear fuel reprocessing and water treatment equipment. Solid RuO4 has been used as a precursor to deposit ruthenium oxide films on polymers substrates at temperatures above 150xc2x0 C.
RuO4(g) is generated by dissolving solid RuO4 in water or by adding a strong oxidizer (e.g., NaOCl, KBrO3, KMnO4, Ce(SO4)2, NaIO4) to RuO2.xH2O or RuCl3 in water. Biological specimens are stained with these ruthenium oxide solutions for microscopy analysis.
Despite the prior art, there are no reports that disclose the use of RuO4(g) to selectively deposit a thin film of RuO2.xH2O on a substrate.
Accordingly, one object of this invention is to provide a method for selectively depositing hydrous ruthenium oxide films on a substrate.
Another object of this invention is to provide a method of functionalizing a surface of a substrate in order to selectively deposit hydrous ruthenium oxide films at the selectively functionalized locations on the substrate surface.
A further object of this invention is to deposit a thin film of hydrous ruthenium oxide onto a substrate.
A still further object of this invention is to deposit a hydrous ruthenium oxide film under ambient or near ambient temperature and pressure conditions.
A further object of this invention is to deposit a hydrous ruthenium oxide film on a substrate at below 90xc2x0 C.
Another object of this invention is to perform such deposition on a range of useful surfaces, including metal and polymer surfaces.
It is a further object of this invention to selectively deposit hydrous ruthenium oxide on a substrate followed by a subsequent metallization for catalyst formation.
It is a further object of the invention to control the degree of structural water content of the deposited RuO2 by heating at selected temperatures up to 175xc2x0 C.
It is a further object of this invention to perform such deposition on flexible substrates, and to have the deposited film have sufficient elasticity to retain conductivity even when slightly bent on flexible substrates.
These and other objects of this invention are achieved in a preferred method of the invention.
An aspect of the present invention is a method for selectively depositing a film of hydrous ruthenium oxide on a substrate, the method comprising the steps of: selectively functionalizing a substrate surface; preparing an oxidizing aqueous solution of a Ru-containing composition; generating RuO4(g) from the oxidizing solution; selectively depositing a film of hydrous ruthenium oxide from the vapor derived from the oxidizing solution on the functionalized surface of the substrate; and depositing by autocatalysis hydrous ruthenium oxide from the vapor of the oxidizing solution on the previously deposited hydrous ruthenium oxide.
Another aspect of the present invention is a method of selectively depositing a film of hydrous ruthenium oxide on a substrate, the method comprising the steps of: selectively functionalizing a substrate surface; preparing an oxidizing aqueous solution of a ruthenium-containing composition, wherein the oxidizing solution contains RuO2.xH2 and Ce(SO4).2H2SO4 or RuCl3.xH2O and Ca(OCl)2 in water; generating RuO4(g) from the oxidizing solution; selectively depositing a film of hydrous ruthenium oxide from the vapor of the oxidizing solution on the functionalize surface of the substrate; and depositing by autocatalysis hydrous ruthenium oxide from the vapor of the oxidizing solution on the previously deposited hydrous ruthenium oxide.